1. Field of the Invention
The present invention relates to a complex filter and calibration method, and more particularly, to a complex filter and calibration method capable of utilizing a small amount of easily integrated compensation resistors to realize phase and amplitude calibration.
2. Description of the Prior Art
In the current wireless communications systems, there are two main architectures of receivers capable of achieving high integration and performing multiple modes. One is low intermediate frequency (low IF) receivers, and the other is direct conversion or named zero-IF receivers. The former can avoid direct current (DC) offset and low frequency noise but may meet image-signal interference. On the contrary, the latter is free from image signal interference but is affected by DC (direct current) offset and low-frequency noise.
Nowadays, low IF architectures are widely applied in transmitters and receivers of the wireless communications systems. Therefore, how to reduce image signal interference in a low IF receiver has become an important issue in the industry and academia community. The most common method in a low IF or very low IF receiver is utilizing a mixer to down-convert radio frequency (RF) signals received from an antenna and obtain a pair of orthogonal signals, and utilizing a complex filter to process the orthogonal signals. Please refer to FIG. 1, which is a schematic diagram of a conventional low IF reception device 10. The low IF reception device 10 is utilized for processing a radio frequency signal RF, and includes a low noise amplifier (LNA) 100, a local oscillator 102, a phase shifter 104, mixers 106 and 108, a complex band-pass filter 110, an analog-to-digital converter (ADC) 112 and a digital signal processor 114. Operating principles of the low IF reception device 10 are well known by those skilled in the art, and thus are briefly illustrated as follows. The LNA 100 is utilized for properly amplifying amplitudes of received signals. The local oscillator 102 is utilized for generating a local oscillating signal for the mixer 106, and the phase shifter 104 is utilized for outputting the local oscillating signal generated by the local oscillator to the mixer 108 after shifting a phase of the local oscillating signal by −90 degree. Therefore, by utilizing the oscillating signals with 90-degree phase difference, the mixers 106, 108 can down-convert the radio frequency signal RF to a specific frequency, and output an in-phase signal I and a quadrature-phase signal Q. The signals I, Q are orthogonal to each other, and mixed with image signals. The complex band-pass filter 110 is utilized for eliminating the image signals within the signals I, Q. Finally, the ADC 112 converts the signals I, Q into digital signals and transmits the digital signals to the digital signal processor 114 for further processing.
In the low IF receiver 10 which separately performs analog and digital operations, an important spirit is that channel selection and image signal elimination are done by the complex band-pass filter 110, i.e. signals are processed under a complex operation architecture, in order to accurately control the signal phases. There are different methods for realizing the complex band-pass filter 110, and one is a leapfrog structure. For example, FIG. 2 is a schematic diagram of a first-order leapfrog complex band-pass filter 20. The first-order complex band-pass filter 20 mainly includes two low-pass filters (also integrators) 200, 202, which are connected by a connection unit including resistors and an inverter. Except for the example shown FIG. 2, multi-order, differential leapfrog complex band-pass filters can also be applied, in order to shift frequency responses of the low-pass filters to a required center frequency, so as to achieve complex band-pass filtering.
In addition, since the low IF reception device 10 is divided into analog and digital operations, if the analog part (i.e. before ADC 112) has phase or gain mismatch, it is hard to completely eliminate the mismatch via the digital part. Therefore, the prior art provides different methods focusing on calibrating phases and amplitudes of the orthogonal signals I, Q, for eliminating image signals. However, most of the conventional calibration methods are established upon complicated computations, and cannot be realized by a small amount of elements or elements capable of being easily integrated into the receivers. Besides, extra complicated computations and excessive elements for eliminating image signals can induce power consumption issues.